Each year many tons of materials such as slag and fly ash resulting from combustion of coal in boilers, hereinafter referred to as coal slag and coal fly ash, found in electric generating plants are produced. In the United States in 1993, for example, over 5.6 million metric tons of coal slag and 43.7 million metric tons of coal fly ash were produced as coal combustion byproducts. The greatest use of such materials is found in roofing granules and as sandblasting materials. Other uses are found in cement and concrete products, snow and ice control, and grouting materials. However, only about 55% of the coal slag and only about 22% of the coal fly ash is incorporated into useful products. The remaining amount is generally disposed of in landfills.
The need to provide additional useful products from such materials and thereby alleviate disposal of these materials in waste storage landfills has long been felt. Forming coal slag or coal fly ash into useful products is considered to be a significant improvement over disposing of such materials in such landfills.
A method for manufacturing rounded vitreous beads is known wherein a feed means which can include a reservoir adapted to hold a fluidized bed of feedstock particles and having at least one overflow under gravity. For manufacture of solid beads, crushed glass cullet is the recommended feedstock; for cellular beads, a pefletized feedstock containing glass formers and cellulating agent is recommended. The feedstock is delivered to the upper end of a chamber which includes a pair of opposed walls which are spaced apart by a distance less than their breadth, e.g., 15-30 cm, and which are angled to the horizontal so that the feedstock can pass through the chamber under gravity.
In this method, a means for heating the chamber is arranged to heat at least one wall of the pair of opposed walls so that feedstock passing between the pair of opposed walls is heated by radiant heat. The chamber can have segments of increased spacing between the opposed wails from the top of the chamber to the bottom of the chamber as well as differing temperatures in different zones downward through the chamber. For some feedstock compositions, it is desirable to allow the particles to expand while subjected to a temperature in the range of 400.degree. C. to 500.degree. C., to heat the particles to 800.degree. C. to 900.degree. C. for spherulization, and to heat them to about 1200.degree. C. for partial devitrification.
A process has also been disclosed for producing ceramic powders based on sine- or multi-phase metal oxides, including SiO.sub.2 compounds, exhibiting a narrow particle size distribution. The raw feed material exhibits a specific surface area of 0.05 up to 500 m.sup.2 /g and is treated in an indirectly heated drop tube furnace in the form of classified granules exhibiting an average diameter of 10 up to 2500 .mu.m for a period of 0.5 up to 15 seconds at a temperature of 500.degree. K. up to 3500.degree. K. The raw feed material flows freely into the furnace via a charging device and drops by action of gravity, in a quasi free-falling manner, through the furnace atmosphere, which is oxidizing, inert or reducing, and is cooled. The cooled-off discharged reaction sintered agglomerates are collected and deagglomerated into primary particles by means of an ultrasonic milling device, a sand mill or a jet mill to produce ceramic powder.
Spheroidal particulate material, such as spheroidal cement or smelter slag can be produced by subjecting the particles to a high temperature flame treatment and rapidly cooling them in air. A retention furnace for retaining a molten liquid inorganic material a a nozzle assembly communicated to the retention furnace and capable of scattering therethrough the molten liquid inorganic material are provided. A jet gas entrains the molten inorganic material introduced in the nozzle assembly and scatters the molten inorganic material to cool it. Gas spray means for spraying the jet gas are also provided.